1. Field of the Invention
The present invention relates to an AD converter, which quantizes input analog signals and converts them to digital signals and outputs them, more particularly, to technical field of the delta sigma type AD converter having band pass filter transmission function.
2. Description of the Related Art
In recent years, public attention has been paid to a digital tuner, which receives analog broadcasting wave and converts intermediate frequency (IF) signals obtained through a high frequency circuit to digital signals so as to digitize subsequent processing. The digital tuner utilizes a delta sigma type AD converter as a component for converting the IF signals to digital signals. Use of the delta sigma type AD converter enables quantization noise in the vicinity of the band of the IF signal frequency to be damped sufficiently, so as to achieve the quantization with a high resolution.
A general broadcasting receiver unit is often constructed so as to be capable of receiving both FM broadcasting and AM broadcasting, and it is preferable to employ the aforementioned delta sigma type AD converter for both FM broadcasting and AM broadcasting by using a common frequency of the IF signal. Further, the delta sigma type AD converter is capable of securing a sufficient damping amount of quantization noise by increasing the order number by multiple connections.
If both FM broadcasting and AM broadcasting are received through a broadcasting receiver unit, a desired frequency characteristic of the delta sigma type AD converter differs depending upon a difference in the modulation type. That is, the bandwidth of the IF signal in case of the FM broadcasting is narrower than the bandwidth of the AM broadcasting. For the reason, when receiving the AM broadcasting, the C/N ratio of the central frequency of the IF signal needs to be sufficiently high. Contrary to this, when receiving the FM broadcasting, although the required C/N ratio of the central frequency of the IF signal is not so strict as that of the AM broadcasting, the quantization noise needs to be damped in a larger bandwidth.
However, in case where a single delta sigma type AD converter is employed for both the FM broadcasting and AM broadcasting, a frequency characteristic preferable for the both cannot be achieved easily. Although there is an over-sampling method for damping the quantization noise in a large bandwidth corresponding to reception of the FM broadcasting, for example, this method induces increase in cost and power consumption of the operational amplifier constructing a delay device and the like because such a structure requires high-speed operation of the circuit. Further, although there is a method for increasing the order number of the digital sigma type AD converter, such an increase of the order number may make feedback loop unstable or increase the circuit size. Further, increasing the order number of the delta sigma type AD converter for the AM broadcasting having a narrower bandwidth than the FM broadcasting produces a lot of waste from viewpoint of the structure.
Accordingly, the present invention has been achieved in views of the above-described problem and therefore, an object of the present invention is to provide a delta sigma type AD converter capable of achieving an excellent performance by providing with optimum frequency characteristic for both FM broadcasting and AM broadcasting without inducing increases in cost and circuit size by devising the setting of the transmission zero point thereof.
The above object of the present invention can be achieved by a delta sigma type AD converter for quantizing an input analog signal, converting it to an output digital signal, and outputting, provided with: a signal processing device which executes filter processing corresponding to a transmission function upon said input analog signal; and a quantizing device which quantizes the input analog signal subjected to said filter processing with a sampling frequency, converts it to the output digital signal, and outputs, wherein the frequency characteristic of quantization noise in said quantizing device has a first transmission zero point on the side of lower frequencies than the central frequency of said input analog signal and a second transmission zero point on the side of higher frequencies of said input analog signal.
According to the present invention, the input analog signal to the delta sigma type AD converter is subjected to filter processing corresponding to a predetermined transmission function. After that, it is quantized with a predetermined sampling frequency, converted to the output digital signal, and output. At this time, the quantization noise Q(Z) added by the quantizing device has the frequency characteristic possessing two transmission zero points above and below the input analog signal. Thus, by adjusting the frequency of these two transmission zero point, the range in which the quantization noise Q(Z) is damped by the frequency characteristic can be widened freely, so that the frequency characteristic preferable for condition such as bandwidth of the input analog signal can be achieved.
According to the present invention, the delta sigma type AD converter is provided with such a characteristic that the transmission zero points are provided above and below the central frequency of the input analog signal. Consequently, the quantization noise in the input analog signal having a wide bandwidth can be suppressed sufficiently without inducing increases of cost and circuit size. Particularly, it is possible to realize a digital sigma type AD converter, which is provided with an optimum frequency characteristic to both receptions of FM broadcasting and AM broadcasting so as to achieve an excellent performance.
The above object of the present invention can be achieved by a delta sigma type AD converter for quantizing an input analog signal, converting it to an output digital signal, and outputting, provided with: a first computing device which supplies a feedback signal to said input analog signal so as to execute a first filter computation; a second computing device which supplies a feedback signal to output signal of said second computing device so as to execute a second filter computation; and a quantizing device which quantizes the output signal of said second computing device with a sampling frequency, converts it to the output digital signal, and outputs, wherein the feedback signal of said first computing device contains a component expressed in xcex1Zxe2x88x921 with respect to a first coefficient xcex1 and the feedback signal of said second computing device contains a component expressed in xcex2Zxe2x88x921 with respect to a second coefficient xcex2, while said first coefficient xcex1 and said second coefficient xcex2 are set to different values from each other.
According to the present invention, the input analog signal to the delta sigma type AD converter is subjected to the first filter computation by the first computing device and then, subjected to the second filter computation by the second computing device. After that, it is quantized with a predetermined sampling frequency and converted to the output digital signal. At this time, the first coefficient xcex1 is set up in the first computing device and the second coefficient xcex2 is set up in the second computing device. The respective feedback signals are constructed so as to contain components xcex1Zxe2x88x921 and xcex2Zxe2x88x921. Then, like the first aspect of the present invention, the delta sigma type AD converter is provided with the frequency characteristic having two transmission zero points by setting the first coefficient xcex1 and the second coefficient xcex2 to different values from each other, so that the frequency characteristic preferable for condition about the bandwidth of the input analog signal can be realized.
In one aspect of the converter of the present invention, said first coefficient xcex1 and said second coefficient xcex2 have a relationship of xcex1=xe2x88x92xcex2.
According to the present invention, the first coefficient xcex1 is set up in the first computing device and the second coefficient xcex2=xe2x88x92xcex1 is set up in the second computing device by the same structure as the second aspect of the present invention. Therefore, the feedback signals of the two computing devices contain components xcex1Zxe2x88x921, xe2x88x92xcex1Zxe2x88x921, which are symmetrical in terms of positive and negative, so that the frequency characteristic symmetrical with respect to the input analog signal can be achieved.
In another aspect of the converter of the present invention, the delta sigma type AD converter is further provided with a changing device which changes over connections of the feedback signal corresponding to said first coefficient xcex1 and the feedback signal corresponding to said second coefficient xcex2 to said first computing device and said second computing device.
According to the present invention, in addition to the structures described in the second aspect and the third aspect, the first coefficient xcex1 and the second coefficient xcex2 can be changed over. Thus, by changing over use of the first coefficient xcex1 and the coefficient xcex2 depending upon the bandwidth of the input analog signal, the frequency range in which the quantization noise is damped is adjusted, so that the performance corresponding to the frequency characteristic possessed by the input analog signal can be optimized.
In further aspect of the converter of the present invention, said input analog signal is an IF signal corresponding to FM broadcasting, while said first coefficient xcex1 and said second coefficient xcex2 are set so that the frequency characteristic of said quantization noise has a damping characteristic in a bandwidth based on frequency modulation.
According to the present invention, the quantization noise can be damped sufficiently in a wider frequency range by adjusting the first coefficient xcex1 and the second coefficient xcex2 corresponding to FM broadcasting of a large bandwidth. Consequently, the FM broadcasting reception performance can be improved without inducing increases of cost and circuit scale.
In further aspect of the converter of the present invention, said input analog signal is an IF signal corresponding to FM broadcasting or AM broadcasting, and said changing device controls itself so as to connect the feedback signal corresponding to said first coefficient xcex1 and said second coefficient xcex2 upon reception of FM broadcasting, while releasing the connection upon reception of AM broadcasting.
According to the present invention, in addition to the operation of the fourth aspect of the present invention, in case of FM broadcasting having a wide bandwidth, the IF signal is changed over so as to use the first coefficient xcex1 and the second coefficient xcex2. In case of AM broadcasting having a narrow bandwidth, it is changed over so as not to use the first coefficient xcex1 and the second coefficient xcex2, so that the quantization noise damping characteristic to both FM broadcasting and AM broadcasting can be adjusted optimally.